In a conventional aspect ratio television picture signal format, such as the NTSC color television picture format for example, extra scan lines are available for carrying useful information. Such information may be vertical interval test information which is useful for monitoring bandwidth conditions of television network paths, or it may comprise digital caption information for the hearing-impaired viewer. In properly adjusted, conventional four to three aspect ratio television displays, the vertical interval scan lines are not visible. Even if the few lines devoted to special services are visible, since the special services information heretofore has not been coherent with the video information, the visibility of these lines carrying special services information has not been objectionable to the ordinary viewer. To those viewers equipped with special receiving apparatus, the special services information has been most useful.
When the raster of the picture display surface is underscanned, portions of the vertical interval, manifested visually as black horizontal bars, appear at the top and bottom of the picture, and any vertical interval information becomes visible within the black horizontal bar typically at the top of the picture image.
Some improved definition or improved quality television systems have been proposed for a wider format picture image having an increased aspect ratio. These changes in aspect ratio have been accompanied by visible scan lines at the top and at the bottom of the fully (properly) scanned picture, sometimes called a "letter box" approach. These scan lines are nominally manifested as dark or black horizontal bands across the conventional picture display.
With an aspect ratio of 1.61, an image transmitted within a conventional 4 to 3 or 1.33 aspect ratio channel (NTSC) frees 40 scan lines per field of information at the top and at the bottom of the picture image. When an aspect ratio of 1.77 is used in these systems, 60 lines per field are unused and are therefore free to carry picture reinforcement information. When the original 10 lines ordinarily available in conventional NTSC signal formats are added to these additional lines, and when an aspect ratio of 1.61 is employed, 200 active picture scan lines and 50 unused scan lines are present in each field. For the ratio of 1.77 there are 180 active lines and at least 60 unused lines. Thus, in the case of the 1.61 aspect ratio, the ratio of used to unused lines is four to one, whereas in the case of the 1.77 aspect ratio, the ratio of used to unused lines is reduced to three to one.
The significance of these used to unused ratios is that in the case of the 1.61 aspect ratio, the information needed to augment all areas of a picture may be carried with a time compression ratio of four; whereas in the case of the 1.77 aspect ratio, the compression required is only by three.
It has been proposed in the prior art to use the blanked lines in the higher aspect ratio systems to carry picture reinforcement information. Picture reinforcement information includes information useful for noise reduction as well as information which is useful for increasing resolution, particularly in scenes which are essentially free of motion over some time period, so that additional details of the image may be carried by multiple successive fields or frames. Sometimes, the additional information carried within a conventional spectrum for image enhancement is known as an "augmentation" signal. The term "reinforcement" as used herein is to be understood as including signals or information relating noise reduction, motion and/or augmentation signals.
The Nippon Television Network Corporation, Tokyo, Japan, (NTV) has proposed to carry high frequency components in the vertical region in order to increase static picture resolution. An article by Yosai Araki, Susumu Takayama, Shuji Hanafusa and Joji Urano entitled "NTSC Compatible Wide Aspect EDTV", published internally within NTV and made publicly available to various committees and organizations, sets forth the NTV proposal in some detail.
The present inventor has proposed in U.S. patent application Ser. No. 07/209,192, filed on Jun. 20, 1988, now U.S. Pat. No. 4,918,515, to carry preemphasis signals within spectral room present within a conventional TV channel in order to increase signal to noise ratios of the television image at the display device of the receiver. This prior patent is hereby incorporated by reference.
An article by William F. Schreiber and Andrew B. Lippman entitled "Reliable EDTV/HDTV Transmission in Low-Quality Analog Channels", SMPTE Journal, July 1989, pp. 496-503, proposes a system employing adaptive modulation of high frequency components and scrambling in order to aid resolution in visually very active areas of the picture and to disperse echoes and channel frequency distortions as random noise in the reconstructed picture. This article suggests that a reinforcement signal, such as a test signal, may be sent through the channel in order dynamically to adapt a correcting filter to channel degradation characteristics at the receiving end of the channel. Echoes and non-linear distortion may thereby be corrected.
On new receivers making effective use of a reinforcement signal or signals carried in the vertical interval, it is easy to cover the dark areas at the top and the bottom of the image (for example by using wider aspect ratio picture tubes). While older receivers following the conventional NTSC format four to three aspect ratio will be compatible with the wider format signal carrying the reinforcement signal, those older receivers will likely display some portions or all of the horizontal black bands at the top and at the bottom of the picture image, and within them the reinforcement signals.
The term "conventional", as used herein in connection with television displays, means that the display has an established prior art conventional display aspect ratio, such as four to three in NTSC, and will compatibly display an enhanced performance television video signal having an extended vertical blanking interval but with the added or extended portion of the interval as dark bands across the top and bottom of the display.
The main drawback of prior reinforcement schemes carried within the visible black bands or bars at the top and bottom of the picture of a conventional display is that the "augmentation" signal is typically coherent in real time with the picture image information being augmented. With a conventional picture tube or other image display device, the augmentation signal is visible and follows in some visible way the activity within the displayed picture image. Rather than remaining solid black or gray bars at the top and bottom on the conventional display device, these bars are now "alive" with light and dark regions and patterns which typically move as the displayed picture image moves and remain stationary when the picture image remains stationary. Thus, the information in the bars is "coherent" with the displayed picture image. As such, the now-visible augmentation signals constitute a major drawback and distraction to the viewer.
While video encryption and scrambling techniques, including reversing the amplitude of video scan lines, have been proposed to protect pay television channel revenues, those techniques have not been applied to the task of hiding or masking otherwise picture-content-coherent reinforcement information within an expanded or extended vertical interval of a downwardly compatible, enhanced performance television signal having a wider aspect ratio than heretofore.
Thus, a hitherto unsolved need has arisen to mask from conventional displays the picture image reinforcement signals carried within an expanded number of "blanked" scan lines appearing as dark bands at the top and bottom of expanded aspect ratio television signal formats on conventional television display devices.